The present application contains subject matter related to that of US Published Patent Application US-2006-0245547, filed Mar. 21, 2006, which is incorporated herein by reference.
Current x-ray imaging systems typically make use of penetrating radiation characterized by a relatively wide-angle pattern that emerges from an x-ray generator such as an x-ray tube. Referring to the prior art configuration depicted in FIG. 1, the angular field of view A of the x-ray beam is conventionally determined by the angular extent P of an x-ray beam 14 emergent from x-ray source 10, in combination with any subsequent collimating structure 12. For example, in the situation depicted in FIG. 1, a wide-angle radiation pattern P emitted by x-ray source 10 and propagating toward the object under inspection 16 is blocked by a highly attenuating material 13 with a stationary collimating aperture 12 that transmits a fraction of the incident radiation in the form of a small fan beam 18. The term “opaque” refers herein to matter that does not effectively transmit the incident radiation. Here, the field-of-view A of x-ray radiation reaching the object 16 is determined by the angular size of the stationary aperture 12 viewed from the x-ray source 10. Referring to FIG. 2, in some cases, x-ray imaging systems may shape the emitted radiation into a scanning pencil beam by means of a chopper wheel 20, or otherwise. In such systems, a continuously moving collimator (or spatial modulator) 20, usually in the form of an opaque rotating wheel with appropriately placed aperture(s) 22, sequentially selects small portions from the wide-angle radiation pattern P emitted by x-ray source 10, positioned at a fixed distance L away from the collimator, and scans the object under inspection (OUI) 16 with a beam B, the transitory position 23 of which on the OUI 16 is accurately knows as a function of time. As used herein and in any appended claims, the term “quasi-collimation” refers to limiting the spatial extent of radiation by means of a single aperture, and, in that sense, beam B is quasi-collimated. As a result of such scan, a backscatter image may be created point-by-point by collecting backscattered radiation from each irradiated pixel for each collimator scan cycle.
For purposes of the current description, a field-of-view (FOV) is defined as the angular extent of an aggregate image comprised by a sequence of transitory illuminating spots formed by an aperture traversing the pattern of penetrating radiation, as viewed from the source. “Imaging” generally refers to generation of a multidimensional representation of values characterizing an aspect of an object or a scene, whether as a stored array or as a displayed representation. “Penetrating radiation” refers to probe radiation, such as in the x-ray portion of the electromagnetic spectrum, which passes into an object, not necessarily traversing the object, and which allows interrogation of various features of the object by virtue of interaction of the probe radiation with the object. “Scanning” a radiation pattern refers to moving a beam of the radiation in a systematic fashion.
“Pencil-shaped,” as used herein, refers to a beam having any cross-sectional shape, the extent of each dimension of the cross-section, transverse to the beam propagation direction, being comparable, though not necessarily equal. “Flux,” as used herein and in any appended claims, refers to either the number, or total power, of x-ray photons crossing a unit cross-sectional area per unit of time.
In prior art scanning x-ray inspection systems of FIGS. 1 and 2, the overall field-of-view, as defined by the span of the radiation-traversing motion of the aperture(s) 22, the angular field-of-view A, is fixed, since it is provided by an x-ray tube's focal spot 11 (shown in FIG. 1), beam forming aperture(s) 12 and 22, and predetermined distance L, all designed to suit a specialized objective. The fixed FOV limits such system to a narrow range of uses, and typically precludes imaging objects outside of a particular design distance, or range of distances, to the OUI 16. An object at a distance shorter than the design distance is “cut-off”, while an object more distant that the design distance suffers resolution loss.